A variety of imaging techniques are used for the medical diagnosis and treatment of patients. Ultrasound imaging represents a prevalent technique. Ultrasound uses sound waves to obtain a cross-sectional image of an object. These waves are radiated by a transducer, directed into the tissues of a patient, and reflected from the tissues. The transducer also operates as a receiver to receive the reflected waves and electronically process them for ultimate display.
Another imaging technique is referred to as Optical Coherence Tomography (OCT). OCT uses light, as opposed to sound waves, to obtain a cross-sectional image of tissue. The use of light allows for faster scanning times than occurs in ultrasound technology. The depth of tissue scan in OCT is based on low coherence interferometry. Low coherence interferometry involves splitting a light beam from a low coherence light source into two beams, a sampling beam and a reference beam. These two beams are then used to form an interferometer. The sampling beam hits and penetrates the tissue, or other object, under measurement, and then reflects from the tissue, carrying information about the reflecting points from the surface and the depth of tissue. The reference beam hits a reference reflector, such as, for example, a mirror or a diffraction grating, and reflects from the reference reflector. The reference reflector either moves or is designed such that the reflection occurs at different distances from the beam splitting point and returns at a different point in time or in space, which actually represents the depth of scan. The time for the reference beam to return represents the desirable depth of penetration of tissue by the sampling beam.
When the reflected beams meet, intensities from respective points with equal time delay form interference. A photodetector detects this interference and converts it into electrical signals. The signals are electronically processed and ultimately displayed, for example, on a computer screen or other monitor.
Obtaining a cross-sectional image of an object involves scanning in both the axial and lateral direction. Typical visual frame rates used in filming moving objects are on the order of 30 Hz. Therefore, to image a moving object, such as a beating heart, for example, a scanner system must be capable of scanning approximately 90,000 data points (assuming 300 data points in both the lateral and axial directions of the object, which is typical for an imaging area of 1×10−4 mm2) in 1/30 of a second. However, to accomplish lateral scanning, many OCT systems utilize reciprocally-moving mechanical parts to move the beam of light across the object being imaged. These moving parts often cannot move quickly enough to complete a lateral scan in the requisite time required by the visual frame rate. Thus, imaging of moving objects, such as a beating heart, will be incomplete from frame to frame. Additionally, the use of such parts creates other obstacles to achieving effective scanning. For example, the inertia of moving parts, and their acceleration and deceleration, causes a non-uniform speed of scan and a reduced speed of data acquisition. Furthermore, vibrations associated with the moving parts may result in additional electronic noise which negatively affects the resolution of scanned images.
Design and manufacture of an effective scanning device utilizing moving parts in combination with medical tools such as, for example, a catheter, also proves difficult. For instance, it is very difficult to control precisely the motion of parts on a tip of a catheter. Furthermore, moving parts generally require more space, thus resulting in an increase in the overall size of the device. For catheters and other similar medical devices, such an increase in size is undesirable.
Certain scanner systems used in the communications industry exploit the ability of certain materials to change properties (such as refractive index) when subjected to the application of an electromagnetic field. These communications scanner systems typically include an optically transparent prism positioned between electrodes. A light beam passing through the prism will be deflected at certain angles depending on the electromagnetic field created by the electrodes and applied to the prism. Because deflection of the light also is a function of wavelength, optimal performance of these scanners requires a monochromatic (single wavelength) light source. These scanning systems therefore typically utilize lasers, which use light having a narrow bandwidth, as the source of light.